A/D converters, which convert analog values into digital values, are widely used in various fields such as measuring, controls, and communications. The demand for accurately converting analog voltage minutely deviating from a reference voltage is growing as the precision of circuits becomes higher (e.g., increase of bit rates of recent A/D converters). One main element that determines the precision of the A/D converter is the comparator which performs comparison between an input voltage and a reference voltage. The comparators used for A/D converters include, for example, a chopper type comparator having a relatively simple circuit configuration.
Although high precision of the chopper type comparator, which is a main element of the A/D converter, is being demanded, one factor reducing the precision of the comparison of the chopper type comparator is offset error caused by, for example, parasitic MOSFETs inside an inverter of the chopper type comparator. As a method for reducing the error, there is proposed a method of reducing offset error by providing a special offset correcting circuit in a chopper type comparator (see, for example, Patent Document 1).    Patent Document 1: Japanese Laid-Open Patent Publication No. 05-235767